136 ELEMENTS OF LABORATORY WORK 



The nature of the residual gas remains to be proved. It is 

 called nitrogen. The gas which has been removed is oxygen, 

 as may be proved afterwards. The density of the residual gas 

 may be found by the method previously used. A weighed 

 flask, containing only a small quantity of water, and of suit- 

 able capacity for the burette, is connected with the nozzle. 

 The tap is turned, and then the clip slowly opened. The 

 nitrogen will enter the flask. The calculations, correcting for 

 temperature, pressure, and water-vapour, are the same as 

 before. 



The density of nitrogen now being known, as well as that 

 of oxygen, it is easy to calculate what would be the density 

 of a mixture of these gases in the proportion by volume 

 observed for air. The result of this calculation should 

 be compared with the observed density of air. They should 

 be found to agree, and afford a reason for supposing the air 

 to be a mixture of oxygen and nitrogen. We have no 

 reason, however, for supposing that this is the exact truth. 

 These experiments cannot be expected to yield anything more 

 than approximate results. Far more delicate investigations 

 are needed to prove the exact composition of air. As far 

 as we know, there may be still some further differentiation 

 possible, although the proportion in which other substances 

 may be present must be small, as is indicated by the close 

 agreement of the calculated density with that which has been 

 observed. 



90. Some Effects of the Atmosphere. The earth is com- 

 pletely enveloped by the atmosphere. The atmosphere or air 

 is therefore present during all natural changes taking place 

 on the surface of the earth ; #nd in many of them it is an 

 essential agent. 



Oxygen is withdrawn from the air, and combines with 

 other matter, in many frequent changes. This combination 

 may take place gradually, as in the case of phosphorus, which 

 has been already described, or rapidly, as when combustible 

 matter burns. Those complex changes which constitute 

 growth and decay can only go on in the air. They are con- 

 ditional on its composition, i.e. the composition of the air 



